Planarity measurements are described, fox example, in German Patent DE-PS 28 13 719 and in German patent Document DE-OS 42 24 569. In both of these patent documents, an indirect measurement of strip planarity is carried out by measuring the strip tension forces across the width of the strip using planarity measurement rollers. The strip tension distribution can be translated into a measure of strip planarity and used in a feedback system to control the strip planarity. The strip is deflected in a measurement direction by a force generating device and the degree of deflection is measured to give the strip planarity. The measuring method is indirect and may be inaccurate because different degrees of deflection of the strip can correspond to length differences in individual strip longitudinal fibers from which the planarity must be determined.
It is possible to carry out a direct measurement of strip planarity on a table or while the strip is at standstill, i.e. when the tension has been removed from this strip. However, this technique is unacceptable for continuous processing lines since it requires an interruption in production.
Planarity measuring rollers, moreover, have a limited accuracy because the longitudinal tension in the strip can only be ascertained with a precision of about 2 to 5 Mpa which leads to inaccuracies in planarity measurement. The transverse stresses are not taken into consideration in this system although they can have an effect on planarity. Furthermore, the earlier measurement systems are sensitive to the influences of neighboring deflecting rollers, drive rollers or the process whereby the strip is wound up, since all of these influence the longitudinal tension distribution across the width of the strip.
Finally, mention should made of the fact that for direct measurement of strip planarity at standstill, the strip can suffer marking as a result of the halting of the travel of the strip. Such marking can itself be unacceptable or can lead to a reduction in quality.